As a typical displacement sensor, the grating measurement system is widely applied to various kinds of mechanical and electrical equipment. The measuring principle of the grating measurement system is mainly based on the moire principle and the diffraction and interference principle. As a maturely developed displacement sensor, the grating measurement system based on the moire principle becomes the first choice of displacement measurement for various mechanical and electrical equipments due to its such advantages as high-resolution, high-precision, low cost, easy installation and adjustment, and etc.
A lithography machine in semiconductor manufacturing equipment is key equipment in the production of semiconductor chip. An ultra-precision workpiece table is a core subsystem of the lithography machine and is used for carrying mask plates and wafers to complete high-speed ultra-precision stepping scanning motion. The ultra-precision workpiece table becomes the most typical kind of system in ultra-precision motion systems due to its such motion characteristics as high speed, high acceleration, large stroke, ultra-precision, multi-DOF (degrees of freedom), and etc. To achieve the above motion, the ultra-precision workpiece table usually adopts a dual-frequency laser interferometer measurement system to measure the multi-DOF displacement of the ultra-precision workpiece table. However, along with the continuous improvements of such motion indexes as measurement accuracy, measure distance, measurement speed and etc., the dual-frequency laser interferometer is difficult to meet the measurement demands due to such a series of problems as environmental sensitivity, difficulty for improving measurement speed, space occupation, expensive price, difficulty for designing, manufacturing and controlling a measurement target workpiece table, etc.
With respect to the above problems, various large companies and research institutions in the ultra-precision measurement field worldwide have launched a series of studies, which have focused on the grating measurement system based on the principle of diffraction and interference, and the research progresses have been published in many patents and papers. Dutch ASML's U.S. Pat. No. 7,102,729 B2 (Publishing date, Aug. 4, 2005), U.S. Pat. No. 7,483,120 B2 (Publishing date, Nov. 15, 2007), U.S. Pat. No. 7,940,392 B2 (Publishing date, Dec. 24, 2009), Publication No. US2010/0321665 A1 (Publishing date, Dec. 23, 2010) disclose a plane grating measurement system and arranging scheme applied to an ultra-precision workpiece table of a lithography machine. The measurement system mainly adopts a one-dimensional or two-dimensional plane grating in cooperation with a reading head to measure large-stroke horizontal displacement of the workpiece table, the height direction displacement measurement adopts such height sensors as eddy current or interferometer, but the application of several kinds of sensors limits the measurement preciseness of the workpiece table. American ZYGO company's U.S. Patent Publication No. US2011/0255096 A1 (Publication date, Oct. 20, 2011) discloses a grating measurement system applied to an ultra-precision workpiece table of a lithography machine. The measurement system also adopts a one-dimensional or two-dimensional grating in cooperation with a reading head to achieve displacement measurement, which can perform horizontal and vertical displacement measurements at the same time. Japanese CANON company's U.S. Patent Publication No. US2011/0096334 A1 (Publication date, Apr. 28, 2011) discloses a heterodyne interferometer. The interferometer uses a grating as a target mirror, but the interferometer can only achieve one-dimensional measurement. Japanese scholar GAOWEI proposes, in the research paper “Design and construction of a two-degree-of-freedom linear encoder for nanometric measurement of stage position and straightness. Precision Engineering 34 (2010) 145-155”, a single-frequency two-dimensional grating measurement system using the principle of diffraction and interference. The grating measurement system can at the same time achieve horizontal and vertical displacement measurements, but due to the use of single-frequency laser, the measured signal is susceptible to disturbances, and it is difficult to guarantee the preciseness.
In order to address the above limitations of the above mentioned technical solutions, a heterodyne grating interferometer measurement system using the principle of optical beat frequency is sought. The measurement system can achieve sub-nanometer or even higher resolution and preciseness, and can at the same time measure large-stroke horizontal displacement and vertical displacement. Adopting the measurement system as an ultra-precision workpiece table displacement measurement device can effectively improve the shortage of a laser interferometer measurement system in the application to the ultra-precision workpiece table, enhancing the performance of the ultra-precision workpiece table of the lithography machine.